The present invention relates to an apparatus for measuring sources of magnetic fields, and in particular, to the apparatus having a SQUID (Superconducting Quantum Interference Device) for detecting magnetism generated from, for example, a living body to be examined.
A technique for sensitively measuring the data of sources of magnetic fields has become an important factor in the fields of, for instance, biomagnetism measurement and resource search. In a measuring apparatus to achieve such measurements, there is often preferably provided a SQUID as a magnetic sensor.
As is known, a SQUID contains a superconducting ring, made of a superconductor, having Josephson junctions inserted in its way and is able to detect magnetic flux as much as 10.sup.-5 to 10.sup.-6 times of a fluxoid quantum -.phi..sub.0 =2.07.times.10.sup.-15 [Wb]. Therefore, feeble magnetic flux caused by pulse currents transmitting among nerve cells can be detected. In particular, the measuring apparatus having the SQUID has been preferably used for research of nerve activities of brains which have received stimulation, such as light and sound.
A conventional apparatus for measuring sources of magnetic fields comprises an element for detecting magnetic flux generated from magnetic sources in a living body to be examined and an element for determining positions of the sources using the detected magnetic flux. The detecting element, as a typical fashion, has pickup coils magnetically picking up magnetic flux from the sources and SQUIDs detecting the picked up magnetic flux. The number of the pickup coils are, for example, 122 channels disposed around a living body. The number of the SQUIDs also is the same.
On one hand, the determining element will receive, through the SQUIDs, data of magnetic flux at the positions of the pickup coils and will be able to calculate the positions of magnetic sources using the data of magnetic flux.
One of the prior methods of calculating the positions of magnetic sources is to draw a distribution map of magnetic fields on the basis of data of magnetic flux picked up through a plurality of pickup coils and to estimate the positions of magnetic sources on the distribution map.
One method on the distribution maps is shown in FIG. 17. In this Figure, a magnetic source (i.e., current dipole) can be estimated to be positioned at the middle point on the line (distance=L) connecting the two peak positions of opposite polarities N and S depicted thereon. The depth of the source may be estimated to be L/2.sup.1/2 at the middle point.
In case that positions of sources are estimated on the distribution map, differences in positions and sizes of pickup coils sometimes causes an error called "local minimum".
Yet another method is to use a performance function. In other words, the positions of magnetic sources is assumed first, then determined is a performance function between the magnetic field distribution due to the magnetic sources at the assumed positions and the magnetic field distribution measured. Then the positions, size and direction of magnetic sources is decided such that the least square error of the performance function be minimized.
However, as measuring for magnetic sources in a patient's head, a plurality of nerve pulse currents, which work as a plurality of sources, extensively exists and each generate magnetic flux. The plurality of sources and their extension make the above estimation methods extremely difficult, because of complex calculation, leading to a large amount of calculation time.
Further, in a conventionally used measuring apparatus, a plurality of pickup coils was arranged so that their axial directions are parallel with each other. It is possible for pickup coils to detect magnetic flux passing them along their axial directions. But, when the direction of currents is the same as the axial direction of pickup coils, it is impossible to measure the positions of magnetic sources.
In addition, as shown in FIG. 18, the axial direction of a pickup coil 2 is aligned with a direction along the depth of an object 1 being examined. Thus, the deeper the magnetic source, the closer the direction of generated magnetic flux to a direction perpendicular to the axial direction; that is, a direction that will decrease measurement sensitivity of the pickup coil 2.
As apparent from the above, the conventional measuring apparatus has a limit in measurable depth; for example, for a head, its shallow portion such as a brain cortex was measurable, while its deep portion such as thaiamuses and so on was immeasurable.